engineered wood on slab ~ glue or float?

We will soon be installing about 1800 sq ft of 1/2"x5" flooring. Our plan was to float it and the manufacturer says floating is okay, but...I have had two salesmen and an installer tell me that they would glue it instead of floating it...Why?

The only clear reason I could get out of any of them was that I have one area where two rooms meet that will be about 35' across. One of the salemen said I would probably have to allow for an expansion joint in this area...Really?

I would like input from someone that knows from experience so if that's you...please offer up your thoughts. Thanks.

unless the slab is near (1/16" or less in 12' or so)) perfectly flat. and you use perfectly flat (not warped) boards. floating will not feel as solid as glueing. but glueing would be a ton harder to remove, when the time comes. thats why i floated. if i "knew" i would never have to remove it, i would have glued.

The main problem with floating the floor is that will be much noisier when you walk on it than if it were glued down. Also there is much more deflection in the floor than a floating laminate would have, like your table may rock when you walk next to it, of course I'm a big guy, so you may not have that problem. I just don't like the way floating hardwood feels to walk on either.